The electronic scanning of documents to sense pencil marks for grading of test papers or reading lists efficiently speeds this type of reading, however the density, opacity, blackness or readability of the marks is sometimes extremely difficult due to the variance in the density of the marks and because of erasures and changes made, or the failure to completely fill in an area to be marked. One common solution for reading such variety of marks is the use of a defocused or low resolution scanning technique to determine where the target area is marked. The average amount of reflected light over the entire target area is used to determine a mark/no-mark condition. This technique allows for the generation of a quality answer for each target area based on the average reflected light over the entire target area, however, using this technique the quality answer for a thin dark mark may be the same as for a smudged erasure.
In U.S. Pat. No. 3,820,068 the background reference level is measured prior to reading and serves as a common comparative for each data channel on the document, however, the use of the background level as a standard does not take into consideration variations of marks from one document to the next.
Another solution is the use of OCR resolution scanning which in effect considers the threshold of both the size and intensity of the marks. This method does not allow for a quality answer and does not accurately process larger variations in mark size and intensity.